1. Technical Field
The present application relates generally to semiconductor memory devices, and more particularly to test systems and methods for detecting transistor defects in semiconductor memory devices.
2. Related Art
Electronic memory devices are well known and commonly found in a variety of electronic systems. For example, electronic memory devices (sometimes referred to as computer memory) can be found in computers and other computing devices. Various removable or stand-alone electronic memory devices are also known, such as memory cards or solid-state data storage systems. For example, it is known to use a removable memory card for storing pictures on a digital camera or for storing movies recorded with a digital video recorder.
Most electronic memory devices can be classified as either volatile or nonvolatile. A volatile electronic memory device is, in general, one which requires power in order to maintain the stored information. An example of a volatile electronic memory device is a Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM) computer memory device, which only retains the stored data while the computer is on, and which loses the stored data when the computer is turned off or otherwise loses power. In contrast, a nonvolatile electronic memory device is, in general, one which is capable of retaining stored data in the absence of an external power source. An example of a nonvolatile memory is a Flash memory. The two main types of Flash memory are the NOR Flash and the NAND Flash.
A typical NOR flash memory cell includes a floating gate transistor as shown in FIGS. 1 and 2. The NOR flash memory cell has a relatively very narrow space between the gate and the drain. However, NOR flash memory devices use relatively high voltages with the memory cells for program and erase operations. In general, process defects located at the gate and drain of a floating gate transistor within a memory cell can easily induce programming failures, erasing failures, and/or some other malfunction. For this reason, semiconductor memory manufacturers usually perform several rounds of pre-cycle tests for the purpose of detecting and screening out as many of such defects as possible, with the goal of reducing failure rates before shipment. Since performing several rounds of pre-cycle tests incurs extra costs and reduces test capacity, a proper test method becomes an important issue for semiconductor memory manufacturers in order to try to balance quality and costs of semiconductor memory devices.
Thus, it is desirable to find new approaches for testing semiconductor memory devices that will allow for improved quality and/or reduction in costs associated with testing processes.